This invention relates to a method and apparatus for producing ion beams for use in semiconductor processing and manufacturing.
In recent years there has been increasing interest in the use of both very low energy (about 500 eV to about 60 keV) and very high energy (about 200 keV to about 3 MeV) ion beams (typically boron, phosphorus, and arsenic) for ion implantation for semiconductor processing. These beams complement the medium energy (about 60 keV to about 200 keV) ion beams that are already widely available for this application. Since space is very limited in semiconductor processing facilities, and because equipment costs are high, a single implanter capable of covering the full ion energy range described above would be extremely desirable.
High energy ion beams may be produced using either tandem accelerators or radio frequency (RF) accelerators. Tandem accelerators have the advantage that they are very energy efficient when compared with RF accelerators. It would therefore be desirable to provide a tandem accelerator for ion implantation over a wide range of ion energies.
In order to achieve the delivery of high currents at low energy with a tandem accelerator-based ion implanter, the invention provides for acceleration of an ion beam in a first accelerating tube to a high voltage terminal, followed by transport of the beam through the terminal without significant charge changing, and deceleration of the beam substantially to ground potential in a second accelerating tube. Since the terminal is maintained at high voltage, the beam optical characteristics between the ion source and the terminal are substantially identical to those of normal tandem operation. The optical elements of the injector and accelerator beamline can therefore be adjusted to produce an optimally focused beam envelope in the high voltage terminal, allowing the beam to propagate efficiently through an empty stripper canal. Since the beam is not stripped in the terminal, it is decelerated in the second tandem accelerating tube. Since the beam propagates through the tandem accelerator at energies higher than the injection energy, expansion of the beam envelope due to space charge and emittance is reduced.
In addition, in this mode of operation the beam optical parameters of the first and second accelerating tubes can be adjusted to further compensate for emittance and space charge effects. The beam reaches the grounded exit beamline of the accelerator with an energy substantially equal to the injection energy. Injection energies can be in the range between about 100 eV to about 250 keV. Thus, means are provided for efficiently delivering a low energy, high current ion beam through a tandem accelerator. To allow efficient transport of the ion beam from the accelerator exit to the process chamber, means are also provided for transporting the beam at an energy somewhat higher than the injection energy and decelerating the beam to substantially the injection energy at a location close to the point of use. Alternately, a somewhat higher injection energy can be used and the exiting beam can be further decelerated to energies below the injection energy near the point of use.